The present invention relates to a treating solution for lithographic printing plates in which silver images are utilized as ink-receptive portions, said treating solution improving ink-receptivity of the image portions at the beginning of printing and inhibiting non-image portions from staining.
Printing plates made using silver halide emulsions having a high sensitivity and capable of being spectrally sensitized have already been put to practical use. Of these printing plates, offset printing plates in which silver images are utilized as ink-receptive portions are disclosed in U.S. Pat. Nos. 3,721,559 and 3,490,905, Japanese Patent Kokoku No. 48-30562, U.S. Pat. Nos. 3,385,701 and 3,814,603, Japanese Patent Kokoku No. 44-27242, Japanese Patent Kokai No. 53-21602, U.S. Pat. Nos. 3,454,398, 3,764,323 and 3,099,209, and Japanese Patent Kokai No. 53-9603.
Methods for making these printing plates are roughly classified into some types, but they are common in that the silver images are rendered ink receptive. A lithographic printing plate comprises an oleophilic image portion which is receptive to inks and a hydrophilic non-image portion which receives water. In usual lithographic printing, both water and ink are fed to the surface of the printing plate to allow the image portion to receive preferentially the colored ink and the non-image portion to receive preferentially water and then the ink on the image is transferred onto a substrate such as paper.
Accordingly, in order to obtain prints of good quality, it is required that the difference of oleophilicity or hydrophilicity between the surface of the image and that of the non-image be sufficiently large.
The above-mentioned methods for making printing plates using silver halide emulsions have favorable features that they are simple, sure and rapid methods and can be automated and furthermore, can provide printing plates of high sensitivity, high resolution and high image reproducibility. However, the difference of oleophilicity or hydrophilicity between the surface of the image and that of the non-image of the resulting printing plates is not sufficiently large as compared with printing plates (PS plate) made using organic colloid such as diazo light-sensitive materials.
As an approach for solving these problems, British Patent No. 1,373,415 and Japanese Patent Kokai No. 58-127928 disclose to improve the ink receptivity by acting a compound having a mercapto group or a thione group and an oleophilic group on silver image. That is, it is considered that the mercapto group of the compound is adsorbed to the silver image and the oleophilic portion of the compound covers the silver image and as a result, the difference of oleophilicity or hydrophilicity between the image portion and the non-image portion becomes relatively large.
However, most of the compounds having mercapto group are water-insoluble or soluble only in alkali solutions since they do not have water-soluble groups such as sulfo group and carboxyl group, and these compounds in weakly acidic region of pH 5-7 required for treating solutions aimed at by the present invention do not dissolve in water, and organic solvents such as ethyl alcohol and isopropyl alcohol have been used for dissolving the compounds.
On the other hand, as a means to inhibit stain of non-image portions, U.S. Pat. Nos. 3,829,319, 4,220,702 and 4,361,639 disclose to improve hydrophilicity of the surface of printing plate by adsorbing hydrophilic colloid particles having an average particle size of 0.1 .mu.m or less such as colloidal silica and colloidal alumina which are inorganic particles onto the surface of lithographic printing plates to form a hydrophilic layer on the surface. That is, the hydrophilic colloid particles such as colloidal silica cover the non-image portion to increase the hydrophilicity of the non-image portion to inhibit the non-image portion from staining.
However, the hydrophilic colloid particles such as colloidal silica have the problem in stability of the solution with time, especially stability at high temperatures. That is, when the solution is stored at high temperatures, the colloid particles tend to precipitate and this tendency sometimes becomes conspicuous depending on the coexisting other components, especially compounds having a mercapto group or the like.
Recently, it is increasingly demanded to reduce use of organic solvents, especially organic solvents having a low boiling point of lower than 100.degree. C. such as methanol, ethanol and isopropanol from the points of environmental problems and safety. For this purpose, for example, it is proposed to solubilize the compounds having mercapto group with surface active agents. The use of surface active agents is considered to be effective as a solubilizing method, but the solublization with active agents has the defect that since the compound having mercapto group is present in the micelle, sufficient ink-receptivity cannot be attained by the compound.